1. Field
The following description relates to an optical transmitter for high-speed optical signal transmission, and more particularly, to an optical transmitter having a phase shift keying scheme, and a method thereof.
2. Description of the Related Art
Due to constantly increasing data traffic such as video and smart phone data, the communication speed used in an optical network is getting faster. In particular, a signal having a speed equal to or larger than 40 Gbps per wavelength appears to meet a bandwidth demand at points where data traffic is concentrated, such as high-performance computing, a server, a data center, an enterprise network, an Internet exchange center, and the like.
Methods of transmitting data that is modulated at a high speed using light include a non-return-to-zero (NRZ) or a return-to-zero (RZ) scheme, in which the amplitude of an optical signal is turned on/off according to an input electrical data, and a phase shift key (PSK) modulation scheme and a differential quaternary phase shift key (QPSK) modulation scheme, in which the phase of an optical signal is modulated.
Methods of transmitting a digital signal that is modulated as described above are divided into a single carrier scheme in which the signal is transmitted over a single carrier, and a multicarrier scheme in which the signal is transmitted over multiple carriers at a significantly lowered speed per carrier.
As for the single carrier transmission scheme, phase shift key modulation in which the phase of an optical signal is modulated to transmit a high-speed signal over a single carrier, and quadrature phase shift key (QPK) modulation and quadrature amplitude modulation (QAM) modulation in which two or more bits are transmitted during a single symbol, may be used. In these schemes, electrical data is directly applied to an optical modulator to modulate the phase of the optical signal.
According to the multicarrier transmission scheme, which is known as orthogonal frequency division multiplexing (OFDM), high-speed data is converted into low-speed data and modulated through QPSK or QAM, and then transmitted in light over multiple carriers by use of fast fourier transform (FFT).
In both of the single carrier scheme and the multicarrier scheme, an optical Inphase-Quadrature (IQ) modulator is used to modulate an optical signal using an electrical signal. By controlling the optical IQ modulator properly, the quality of the modulated optical signal is maintained. Accordingly, an optical transmitter provided with an optical IQ modulator needs to maintain an optimum operating point regardless of surrounding temperature while supporting the single carrier scheme and the multicarrier scheme.